When rebasing from the UI, add a system note
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As part of reviewing the docs change in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182013, I asked myself "why don't we use the word **rebase** in the system note when a user triggers a rebase from the UI?"
We can't always know _from Git_ if a push contains a rebase, but if the user triggers the `/rebase` quick action or chooses to rebase from the UI, we _do_ know. It'd be nifty if the system notes reflected the difference. Right now, they don't:

The system note for rebases from the UI add two extra words: `[user] rebased and added [$num] commits [$timeago]`.
This would also improve the UX a tiny bit for the forgetful among us (hi, it me) who often wonder "did I intend to rebase and forgot, or did I really do it?"
## Related issues
Related to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182013+ which finally made me ask the question
issue